The subject of the invention is a process for the complex preparation of markings or marking systems, through the modification of the structure or internal stress within materials, and for the non destructive reading of these markings or marking sets. It provides a complex system for marking and the reading of the markings, thus it is a process on the one hand, and an equipment system on the other hand, to produce marks, marking through the modification of the structure (texture), or local stress in the layer near the surface of materials by high energy density (HED) surface treatment, respectively. In the course of the process the information is applied with the help of a beam targeted (concentrated) to the surface, with the power density adjusted to a level ensuring irreversible change at least in the structure of the surface layer of the material. The markings can be read in a non-destructive and contactless manner through the measurement of electric, or acoustic characteristics and the information contents (coded) or position (localisation) of the markings can be determined with the aid of special electronics or measuring system.
WO-A-94/11146 discloses a system for tracking metallic objects by the application and subsequent reading and comparison of information incorporated therein with stored data, the system comprising a device for applying information to an object along a prescribed pattern, storing devices and a device for reading the information from the marked object. The markings are produced by transferring heat energy along the prescribed pattern using a laser permanently changing the surface layer of the object to a prescribed depth. The information reading device contains an energizing device energizing the surface layer of the marked object with light and an optical detecting unit for detecting a response signal invoked at the marked object by said energizing light and a unit for restoring information from deviations caused by local unevenness of said response signal. These markings are often suitable and the users achieve their purpose by using them but in several cases are not adequate as they can be easily falsified.
In many cases it may be important for the information not easily readablexe2x80x94that is the information should be concealed. In other cases the traditional visual (optical) reading can not be used reliably due to the environmental conditions (e.g. pollution, corrosion). The aim of the invention is to develop a system for the tracking metallic objects by applying and subsequently reading of marks, markings incorporated into or bound to said objects and comparison of read information with recorded data, which system comprises a marking device applying information to the object through a prescribed pattern, record keeping devices and a device for reading the information from the marked object.
The process provides furthermore information about different methods of applying markings carrying information, discloses methods for reading instruments applicable therefor as defined in the appended claims.
The preparation of marks, markings or marking sets of known characteristics through high energy density surface treatment (e.g. laser) of the near surface layers of materials. The markings carry analogue or digital information that can help identification, are coded (e.g. indicating serial number with barcode), or determine geometric place (e.g. distance units of length measurement). The markings can be invisible (e.g covered by paint, embedded in plastic or paper, or can even be under corroded surfaces). The markings can be read with or without any air gap, with stationary or moving reading head. The coded information of the markings can be decoded with the aid of a special electronic device. The prescribed spacing can be determined with a special measuring instrument.
The present application offers methods suitable to solve these tasks in a sophisticated manner and compiled into systems.
In the cases explained in the application the information is provided by the remanent modification of the material structure caused in the near surface layer, and through the readability of its discernible magnetic, electric or acoustic characteristics. It is expedient to use structural changes that can be measured (read) in a scanning mode and without the destruction or significant weakening of the signal even through a coating (paint, corrosion) without any contact in a non-destructive manner. The structural change can be different for different materials (e.g. for steel change in the texture, residual stress state and thus in the magnetic structure is also suitable).
With the local changes in the structure and/or residual stress state in the near surface layer of the material, the magnetic, electric and acoustic properties also exhibit a local deviation, respectively.
With the introduction (or removal) of local heat energy as a result of the changes caused by it, (residual under the conditions of application), local material creep (plastic deformation), local residual stress changes can be induced in all solid materials.
If the solid matter can exist in different structural states (phases, grain, allotropic form), by applying the treatment with the required energy density it is possible to induce local structural changes.
It is well known, that magnetic electric properties like permeability, coercitive force, etc. of magnetizable materials depend on the structure of the material and the stress.
Thus, for magnetizable materials the local change of the stress will result in the local change of magnetic properties.
For instance, for low carbon steels (0.1%) the Barkhausen noise (BN) in the stress state near the flow limit can be the multiple of that measured in stress free state.
In magnetizable materials the locally modified material structure (or in non-magnetizable materials the locally modified magnetic material structure) will result in locally changing magnetic properties.
Example for structural steels (0.2-0.8 C%), it is possible to induce zoned, martensitic transformation through a surface heat treatment carried out by a laser with high enough power density and caused by the high speed internal heat conduction ensured by the material itself. The magnetic Barkhausen noise measured under the same conditions on the zone of martensitic transformation is only a fraction of that of the base material, thus the geometric location and size of the annealed zone is easily detectable on the basis of Barkhausen noise measurement.
Example: As a result of local treatment, ferromagnetic changes occur in the corrosion resistant austenitic steel which (clearly) results in measurable magnetic Barkhausen noise value.
It follows from the above, that with the measurement of magnetic properties it is possible to arrive at deductive conclusions about the stress state and its changes, on the basis of which the place and certain characteristics of the treatment can be determined.
Local changes in the material structure, attained through a treatment carried out with enough power density will induce local changes in the electric conductivity of conductive materials, allowing thus the place and certain characteristics of the treatment to be determined.
Changes in the texture and mechanical stress causes changes in the acoustic characteristics (velocity, decrease).
Markings can be prepared characteristically with high energy density (HED) processes. These include processing by laser, plasma, ion, electron and focused light beam (depending on the desired geometric characteristics and material), but any other method can be contemplated which can produce local structural changes trough its power density causing intensive heating or cooling.